Effects of soil and water conservation practices on selected soil physico-chemical properties in Debre-Yakob Micro-Watershed, Northwest Ethiopia

Although different types of soil and water conservation practices ( SWCPs) were introduced, the sustainable use of these practices is far below expectations, and soil erosion continues to be a severe problem in Ethiopia. There-fore, this study was conducted at Debre Yakobe Micro-Watershed (DYMW), Northwest Ethiopia with the general objective of finding out the effects of SWCPs on selected soil physico-chemical properties. Practices including non-conserved plot in the gentle slope (NCGS), non-conserved plot in the moderate slope (NCMS), bund with Sesbania sesban in the gentle slope (BSGS), bund with Sesbania sesban in the moderate slope (BSMS), bund with pigeon pea ( Cajanus cajan ) in the gentle slope (BPGS) and bund with pigeon pea in the moderate slope (BPMS) were examined. Soil samples were collected from different land management plots and soil texture while bulk density (BD), p H, organic matter (OM), total nitrogen (TN), Available Phosphors (Av.P), and cation exchange capacity (CEC) were analyzed in the laboratory. Data were analyzed with the use of one-way analysis of variance and simple regression methods. The results of the study revealed that there is significant difference (p< 0.05) between conserved and non-conserved plots. The conserved plots had the highest soil pH (BPMS) 6.7, OM (BPMS) 3.01%, TN (BSGS) 0.13%, Av.P (BPMS) 4.20 (ppm) and CEC (BSGS) 32.01 cmol(+) kg -1 but a lower BD (BPMS) 1.34 g cm -3 ; suggesting that bund accompanied by Sesbania sesban and pigeon pea were found to be effective in improving soil physico-chemical properties in the study area.


INTRODUCTION
Erosion, one of the symptoms of unsustainable land management, is a consequential degradation process affecting the soil resource in the entire world (Herweg and Stillhardt, 1999). It is a severe problem in the highlands of Ethiopia, especially in the Amhara Region (Lakew Desta et al., 2006).
The result of erosion hazard assessment by Betru Nedasa (2003) indicates that about 6.4 million hectare (ha) or 38% of the Amhara Region's area suffers from high to very high erosion hazards caused by water. Sheet, rill and gully erosion are commonly observed in the high rainfall areas of East and West Gojam where Nitosols are dominant (Birru Yitaferu, 2003). These types of erosion carry away the fine soil particles of the most fertile topsoil and organic matter (Gete Zeleke, 2000).
Poor soil management and land use practices are the causes of severe soil erosion rates in Ethiopia

Description of the study area
Debre Yacob Micro-Watershed (DYMW) is located in Mecha district (38 km south west of Bahir Dar city), within Koga watershed between 11⁰ 10̕ ̕ 06" to 11⁰ 24̕ 22" N and 32⁰ 02̕ 48" to 37⁰ 17̕ 41" E ( Figure 1). Its area coverage is 325 ha. The average annual rainfall (RF) is 1300 mm and the mean monthly temperature is 20 0 C. The elevation ranges between 2074-2262 meter above sea level (masl), and the slope ranged from flat to very steep. Agroecologically, the micro-watershed is known as Woyna Dega (mid altitude). The total population of the micro-watershed is 975 (male 497 and female 478) and the average family size is 4.6 (Fisseha Moges and Habtemariam Assefa, 2017 Agriculture is the main stay of livelihood in the study area. Crop and livestock production are fully integrated and thus the production system can be referred to as crop livestock mixed farming system. Teff (Eragrostis teff), maize (Zea mays), sorghum (Sorghum bicolour) and Millet (Eleusine coracana) are the major crops grown in the micro-watershed.

Soil sampling technique
A reconnaissance survey was carried out to have a general view about the study area. Following the general site selection, representative soil sampling sites were selected both from the farm plots where integrated physical and biological SWCPs have been practiced and plots with no SWCPs (cultivation land adjacent to each structure). In each selected plots, soil samples were collected from the top 0-20cm depth at four corners and at the center of a plot to obtain representative composite sample per treatment.

Soil Analysis
The analysis of physical and chemical properties of soil was carried out at Amhara National Regional State (ANRS) Soil Research center.
The surface soil samples collected from the study area were air dried, crushed and sieved to pass through 2 mm sieve for the analysis of pH, particle size distribution, CEC, exchangeable cations and available P and through 1 mm sieve for the determinations of organic matter and total nitrogen. Particle size distribution was analyzed by the modified Bouyoucos hydrometer method (Bouyoucos, 1962) using sodium hexametaphasphate as dispersing agent. Bulk density was estimated from undisturbed soil samples collected using core sampler, and the relationship is: Bulk density (g·cm -3 ) = Oven dry soil mass (g)/Core volume (cm 3 ).
The pH of the soil was measured potentiometrically using a digital pH meter in the supernatant suspension of 1: 2.5 soil to liquid ratio where the liquids were water (Van Reeuwijk, 1992). Organic  Total Nitrogen (TN) was determined using the Kjeldahl digestion, distillation and titration method (Black, 1965) whereas the Olsen's extraction method Olsen et al. (1954) was used for available phosphors extraction under wide range of pH (Landon, 1991;Tekalign Mamo and Haque, 1991).
Cation exchange capacity (CEC) was determined from ammonium acetate saturated soil samples that were subsequently replaced by Na from a percolated sodium chloride solution. The excess salt was removed by washing with alcohol and the ammonium ion that was displaced by sodium was measured using Kjeldahl procedure (Chapman, 1965) and reported as CEC.

Method of Data analysis
The data obtained from laboratory and field measurements were analyzed using one way analysis of variance (ANOVA) using SPSS, 16 (2007). Mean comparisons between treatments were conducted using LSD pos hock test method and linear regression analysis using SAS 9.1.3 to quantify some correlations between soil properties.

Practices on Soil Physical Properties
Soil texture: Table 1 shows that there was significant difference between sand and clay content (p< 0.05, R2 of 0.68 for sand and 0.71 for clay). Specifically, among the treatments, soils of non-conserved plot had the highest mean value of clay and the lowest sand (p< 0.05) content. There was no significant difference among treatments that were managed through different SWCPs. (2010)  Bund stabilized with pigeon pea in the gentle slope showed significant (p< 0.05) means variation with bund stabilized with Sesbania sesban in the gentle slope. Table 1 indicates that though there was significant soil texture difference between conserved and non-conserved farmlands, the texture class showed that all treatments were clay soils.

Bulk Density
The non-conserved plot was found to exhibit significantly the highest mean value of bulk density than plots treated with SWCPs (Table1).

Soil Organic Matter: Soil organic Matter (SOM)
showed significant variation R 2 = 0.87 (p < 0.05) with respect to treatments. The SOM content under the non-conserved plots was significantly lower than all the other treatments ( Table 2).

The significantly lowest SOM occurred in non-
conserved plot (2.4%), while the highest (3.01%) SOM showed in the plot which was conserved with Bund + pigeon pea + gentle slope (Table 2) The result agrees with the finding of Yihenew Soil P H: The mean of soil pH significantly varied between treatments (P< 0.05). The overall p H value of the study area ranged between 6.03 and 6.66. It was lower in moderate slope control farm land and higher in the bund found at gentle slope stabilized with pigeon pea (Table 2). Though there were significant differences between treatments with gentle and moderate slope gradients, the soil P H was within the preferred range for most agricultural practice, 5.5-7.0 P H (Brady and Weil, 2002).

CEC:
The analysis of variance showed that the overall mean CEC values did not significantly vary (p< 0.05) with respect to treatments and slope gradients ( Table 2). The variations between treatments as well as slope gradients were very small. This could be attributed to the soil texture, particularly to clay. This is in line with Gebeyehu Tilahun (2007) who reported that CEC is significantly and positively correlated with clay. In surface horizons of mineral soils, higher OM and clay contents significantly contribute to the CEC, while in the subsoil particularly where Bt horizon exist, more CEC is contributed by the clay fractions than by OM due to the decline of OM with profile depth (Foth, 1990;Brady and Weil, 2002). These finer soils (clay) are negatively-charged particles.
For this reason, they can attract, hold and release positively-charged nutrient particles (cations). As a result, higher clay containing soils can hold more exchangeable cations than soils having low clay (Alemayehu Assefa, 2007). Therefore, this could be the reason for the insignificant difference between treatments among CEC values.
Based on the above ratings, both conserved and non-conserved treatments in the study area qualify to the higher status of CEC (Table 2).

Effects of SWC Measures on Crop Yields
In